The invention disclosed herein pertains to a process for making a ferritic spheroidal graphite iron melt which is in condition for direct casting and does not require annealing for converting the carbon content of the melt to spheroidal graphite in ferrite as is sometimes done conventionally.
It is known that cast iron constituted primarily by a matrix of ferrite or pure iron in which there are isolated nodules or spheroids of pure graphite results in castings that are elastic, tough, ductile and resistant to fracture when sharply impacted. A method of producing ferritic spheroidal graphite iron that has been used for decades involves first making a cast iron melt and then pouring the molten metal into molds to make castings. At this point, as is known, some of the carbon will be chemically combined with iron as cementite, Fe.sub.3 C. This is extremely brittle and hard. To make the metal more ductile and malleable, it is annealed at temperatures in the range of 800.degree. C. to 1000.degree. C. (1470.degree. F. to 1830.degree. F.) which decomposes the iron carbide, leaving iron and isolated carbon nodules which have some degree of the desired sphericity.
By this historic process, annealing is done for hours and even days. Energy expenditure is high and, therefore, the cost of the malleable/ductile iron castings is high. The cost is also increased as the result of a fairly large number of castings having defects such as gas pockets. Malleable/ductile castings also warped so they had to be subjected to a costly straightening process.
More recently developed methods of making ductile spheroidal graphite iron castings require a high proportion of pig iron in combination with scrap steel. A good grade of pig iron is one that has low manganese, low tramp element and low sulphur content. Manganese promotes retention of carbon in the combined form of pearlite which is detrimental to formation of the ductile grades of spheroidal graphite. On the other hand, manganese has the beneficial effect of combining with sulphur to thereby prevent iron from combining with sulphur. Iron sulphide reduces ductility of iron. The best grades of ductile iron available today require a furnace charge that is low in sulphur and manganese and has controlled levels of silicon and carbon. In current practice, the furnace charge for a batch of ductile iron usually comprises scrap steel, pig iron and foundry returns. Returns are the pieces of metal such as sprues, gates, risers from the molds and scrap castings. Typically, practitioners of the best process developed up to the present time, use a ratio of pig iron to scrap steel in the furnace charge which is between 1:3 when using consistently high quality low manganese scrap and 3:1 when the steel is of uncertain origin. Low manganese (Mn) content pig iron contributes carbon to the melt and dilutes the manganese and because of the low sulphur content, reduces the amount of magnesium inoculant required to combine with sulphur before the melt is poured. Low manganese pig iron is available but is very expensive. A process that permits making as-cast ferritic spheroidal graphite iron which avoids using any pig iron and allows use of readily available steel scrap is recognized as being highly desirable.
The use of steel scrap of unknown analysis in known ductile iron melting processes creates problems to which attention must be given. Much of the available steel scrap consists of alloyed metals from the bodies and other parts of automobiles. The sheet metal used for automobile bodies in recent years is rolled out thinner than formerly to reduce the weight of automobiles. Reducing the thickness of the sheet metal cannot be done at the expense of strength. Hence, alloys are being used extensively to compensate for the loss of strength that would otherwise result from reducing thickness. Using more exotic alloys for automobile and other machinery parts has resulted in scrap steel containing higher Mn and other tramp elements which are detrimental to yielding spheroidal graphite in the ferritic condition which is associated with ductility. Foundry experts have found that the effects of undesirable alloy and impurity components such as chromium, titanium, phosphorus and other metals and an excess of manganese which is now found in much of the scrap steel, can be offset to a large extent by using substantial amounts of high grade low sulphur and low manganese content pig iron in the melt. The cost of this pig iron is exacerbated by reason of much of it having to be imported from either Canada, France, Brazil or the USSR. As a general rule, foundries increase the amount of pig iron in a melt in correspondence with increasing manganese in the steel scrap used in the melt because, as mentioned, manganese promotes pearlite and inhibits transformation of carbon to spheroidal graphite.